1. Field of the invention
The invention relates to minimally invasive and non-invasive clinical testing. More particularly, the invention relates to an apparatus and method for modifying localized absorption and scattering coefficients at a tissue measurement site during optical sampling.
2. Description of the Related Art
Conventional methods of clinical testing have required the use of invasive procedures, such as biopsy and phlebotomy, to sample blood and tissue. Subsequently, the samples were transported to a central location, such as a laboratory, for examination and analysis. There is an increasing trend, however, toward point of care testing and even in-home testing. One of the benefits of this trend is to minimize the turnaround time from when a sample is taken to being able to take action based on the test results. At the same time, sampling procedures are becoming less and less invasive. Since they minimize or eliminate the need to handle blood and tissue specimens, minimally invasive and noninvasive procedures drastically reduce biohazard risk, both to the subject and the practitioner. Additionally, the decreased use of expendable reagents minimizes cost of testing and the environmental and health risks posed by the use of chemical substances.
Analyzers are being developed for point of care and in home use that either sample in a minimally invasive fashion or are completely noninvasive, often by sampling tissue optically. During use, it is necessary for many of these analyzers to contact the surface of a tissue measurement site directly, in order to control test conditions such as:
stability of the analyzer during measurement;
minimization of spectral reflectance;
avoidance of stray light; and
reproducibly hitting the targeted sampling area.
Pressure on the sampled tissue (skin) site induced by contact with the analyzer can result in localized sampling variations. For example, pressure applied to the tissue measurement site forces water from the vicinity of the site, decreasing the water concentration. As water concentration changes, there is a corresponding change in the local absorption coefficient. In addition, decreasing water concentration increases the density of the scattering centers present in the sampled tissue volume, thereby altering the reduced scattering coefficient. It would be desirable to modify local absorption and reduced scattering coefficients in a controlled, reproducible manner, allowing differential measurements to optimize the signal-to-noise ratio of one or more target analytes.
It would also be advantageous to provide sampling devices that either maintain a constant pressure or displacement between the analyzer and the subject""s skin or that reproducibly control changes in pressure or displacement over time.
The invention provides a subject interface module for modifying localized absorption and scattering coefficients by controlling the pressure applied to a tissue measurement site by an analyzer during optical sampling; the applied pressure may be maintained at a constant level, or it may be applied in a controlled, reproducible manner as a function of time, so that absorption and reduced scattering coefficients may be varied in a controlled, reproducible manner. The invention is also embodied as a method of modifying localized absorption and scattering coefficients in a controlled and reproducible manner by varying pressure or displacement during optical sampling.
The preferred embodiment of the invention includes a placement device for receiving a body part such as an arm, so that the body part is held in a fixed position and at a fixed elevation. The invention further includes an applied force mechanism for advancing the fiber optic probe of an analyzer until it makes contact with the body part, and maintaining the contact at a constant pressure. The applied force is supplied by a counterweight on a single arm balance. The invention further provides a temperature control, for equilibrating the temperature of the fiber optic probe with the surface temperature in the immediate vicinity of the tissue measurement site.
Alternate embodiments of the invention provide a means for bringing the fiber optic probe into contact with the surface of the tissue measurement site, and then displacing it by a known distance. In one embodiment, an LED and a detector define a starting location prior to displacement and the fiber optic probe is displaced a given distance after the LED is detected. In another embodiment, the displacement of the probe is dictated by the elimination of spectral reflectance. In a further embodiment, the probe is displaced into the tissue until analysis of the spectral information indicates that the preferred depths of the sample are being probed.